how to write a function pointer "pointing" to a macro which expands to the actual function?

Question

I have function in library lib.so which I'm linking to my application dynamically using dlopen()

lib.h

void DebugPrint( unsigned char logLevel,
                 const char     *programName,
                 const char     *format,
                 ... );

#define DBG_PRINT(logLvl, format,  ...)             \
        DebugPrint(logLvl,TL_MODULE_NAME, format, ## __VA_ARGS__)

myapp.c

void (*DBG_PRINT_ptr)( unsigned char logLevel,
                 const char     *programName,
                 const char     *format,
                 ... );

    void *handle = NULL;
    bool ret = RESULT_SUCCESS;

    /* Open Shared Lib into the same Process */
    /* LDRA_INSPECTED 496 S */
    handle = dlopen(lib.so, RTLD_NOW);
    if (NULL == handle)
    {
        /* fail to load the library */
        LLOG_error(" dlopen Error to open handle: %s\n", dlerror());
        ret = RESULT_FAILURE;
    }
        if(RESULT_SUCCESS == ret)
    {
              DBG_PRINT_ptr = dlsym(handle, "DebugPrint");

        if( DBG_PRINT_ptr  == NULL)
        {
           LLOG_error("Failed in DBG_PRINT dlsym(): Err:%s", dlerror());
           dlclose(handle);
           ret = RESULT_FAILURE;
        }
   }
(void)DBG_PRINT_ptr  ("loglevel1","dd","printval");

but I'm getting error while runtime Failed in DBG_PRINT dlsym(): Err:Symbol not found

what is the correct way to define function pointer for the following requirement.

I don't see how the macro is relevant to your question as you aren't using it anywhere. — Kevin, Apr 28 '20 at 16:18
Does this answer your question? [Explicitly exporting shared library functions in Linux](https://stackoverflow.com/questions/2164827/explicitly-exporting-shared-library-functions-in-linux) — Marek R, Apr 28 '20 at 16:35
Be aware that C++ allows for function and operator overloading. Some linkers use *name mangling* to map this overloaded functions to a C style function name. — Thomas Matthews, Apr 28 '20 at 17:10
Try `nm -D lib.so` to find out if your shared lib actually exports this symbol. — Lorinczy Zsigmond, Apr 28 '20 at 19:57

eerorika · Answer 1 · 2020-04-28T16:34:57.650

1

There is no way to point to a macro with a function pointer. Only functions can be pointed by a function pointer.

You can have a pointer to the function that the macro calls however. Like this:

auto fptr = &DebugPrint;

Symbol not found

This means that there is no symbol by that name in the dynamic library.

One typical mistake is to attempt to load a function which has C++ language linkage. The symbol of such function will be mangled and won't match the name of the function.

Functions can be declared to have C linkage with a language linkage declaration:

extern "C"

edited Apr 28 '20 at 16:34

answered Apr 28 '20 at 16:23

eerorika

232,697
12
197
326

tittle was wrong code do not try access macro in dll. – Marek R Apr 28 '20 at 16:26
since I'm using the c language, I cannot use auto keyword for function pointer – mahesh hegde Apr 28 '20 at 17:08
@maheshhegde Yeah, other languages are different. My answer is about C++. – eerorika Apr 28 '20 at 17:11
@maheshhegde: if you don't want C++ answers, don't use the [tag:c++] tag. I removed it for you. – rici Apr 28 '20 at 17:49

how to write a function pointer "pointing" to a macro which expands to the actual function?

1 Answers1